The present invention relates to polyester resins especially useful in vapor permeation curable coatings and more particularly to an improved synthesis route for making phenol-functional polyester resins.
Vaper Permeation Cure (VPC) is a method for curing polyhydroxy polymers with a multi-isocyanate curing agent by exposure thereof to a vaporous tertiary amine catalyst at room temperature. VPC characteristics include extremely rapid cure times often ranging from as low as about 15 to 30 seconds and the ability to effect complete polymer cure at room temperature. Exemplary uses of VPC which take advantage of these properties include the curing of foundry binder compositions (U.S. Pat. No. 3,409,579) where the speed of cure is important and in coating heat-sensitive substrates such as thermoplastic substrates (commonly assigned application of Blegen, U.S. Ser. No. 216,323, filed on Dec. 15, 1980). Of course, other particular adaptations of VPC technology exist as will be readily apparent to those skilled in the art.
A typical VPC coating composition comprises a polyol in admixture with a multi-isocyanate curing agent. Aromatic hydroxyl-functional resins are preferred for this purpose. For example, U.S. Pat. No. 3,789,044 proposes a VPC curable coating composition wherein the resin is made by reaction of hydroxybenzoic acid with an epoxy resin. The phenol-functional reaction product then is admixed with the multiisocyanate and cured by exposure to a vaporous tertiary amine catalyst. U.S. Pat. No. 3,836,491 proposes to cap a polyester resin with a hydroxybenzoic acid which phenol-functional polyester is admixed with a multi-isocyanate curing agent and cured according to VPC techniques. With regard to the former patent, unfortunately the residual aliphatic hydroxyl groups from the oxirane rings in the epoxy resin diminish the pot life of the coating composition and retard the time to ultimate cure which is experienced when VPC cure is practiced. As to the latter proposal, it has been discovered that hydroxybenzoic acids are subject to decarboxylation when used in an attempt to form the phenol-functional polyester at typical polyesterification temperatures of greater than about 200.degree. C., e.g., typically about 235.degree. C. Thus, as regards hydroxybenzoic acid-capped polymers for use in VPC coatings, there exists a need in the art for overcoming the foregoing shortcomings of prior proposals.